Steel sheet for textile machinery parts and method for manufacturing the same

ABSTRACT

A steel sheet for textile machinery parts manufactured at a low cost and is excellent in wear resistance and toughness. The steel sheet for textile machinery parts contains, in mass %, 0.60% or more and 1.25% or less C, 0.50% or less Si, 0.30% or more and 1.20% or less Mn, 0.03% or less P, 0.03% or less S, 0.30% or more and 1.50% or less Cr, and 0.10% or more and 0.50% or less Nb, with the balance being Fe and unavoidable impurities. Furthermore, Nb-containing carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 3000/mm 2  or more and 9000/mm 2  or less. Nb-containing carbides&#39; effect of improving wear resistance can be ensured, and deterioration in toughness due to excessive formation of Nb-containing carbides can be prevented. Hence, the resulting wear resistance and toughness are good.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase application under 35 U.S.C. § 371 of International Patent Application No. PCT/JP2015/070444, filed Jul. 16, 2016. The entire contents of these applications are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The present invention relates to a steel sheet for textile machinery parts excellent in wear resistance and toughness and a method for manufacturing the same.

BACKGROUND

Textile machinery parts to be used for knitting machines, such as latch needles, needle plates, sinkers, selectors and jacks are required to have wear resistance, so that in general, quenched/tempered high carbon steel materials are used. These textile machinery parts are subjected to abrasive wear due to contaminants contained in yarn, such as Al₂O₃ and SiO₂.

And in recent years, development of costume made of dense knitted fabric tends to result in thinning of textile machinery parts. If textile machinery parts are thinned by wear, the knitting positions are shifted. Hence, steel sheets for textile machinery parts are required to have even more improved wear resistance.

Meanwhile, when a knitting machine is in high-speed operation, various parts may be broken by the impact upon the reciprocating-sliding motion of textile machinery parts.

For example, when a latch needle is broken, fabric being knitted is damaged by a broken portion of the latch needle, causing a problem in terms of the commercial value of the fabric.

Further, when a malfunction takes place in a selector that is a part for selecting yarn, the selector collides with a needle plate to break the needle plate. The needle plate is fixed to the main body of a knitting machine while being held by several wires, and thus the broken needle plate cannot be easily exchanged.

Accordingly, in order to prevent various parts from breaking due to the impact from the reciprocation of textile machinery parts, toughness against impact from such reciprocation is currently ensured by lowering the level of hardness although the wear resistance is decreased.

For example, Japanese Laid-open Patent Publication No. 59-43128, Japanese Laid-open Patent Publication No. 62-89841, Japanese Laid-open Patent Publication No. 4-88149, and Japanese Laid-open Patent Publication No. 5-171355 describe textile machinery parts excellent in strength, toughness, and anti-corrosion characteristics, which are used for felt needles, sewing machine needles, latch needles and the like. In aforementioned patent references, medium carbon steel is used as a base and Cr, Mo, V and the like are added, so as to improve wear resistance and the use life.

Furthermore, Japanese Laid-open Patent Publication No. 2000-192197, Japanese Patent Publication No. 3946370, Japanese Laid-open Patent Publication No. 2001-181799, Japanese Laid-open Patent Publication No. 2002-220640, Japanese Patent Publication No. 4789225, Japanese Laid-open Patent Publication No. 2002-285287, Japanese Laid-open Patent Publication No. 2002-285350, Japanese Patent Publication No. 4420176, and Japanese Laid-open Patent Publication No. 2009-203528 describe stainless steel to be used for weaving machine members. In aforementioned patent publications, martensite-based stainless steel is used as a base and the total amount of precipitated carbides such as Ti and Nb is specified, so as to highly strengthen the steel and suppress the wear of the steel sheet that comes into contact with fibers. Moreover, Cr forms a passive film, so as to improve anti-corrosion characteristics.

SUMMARY

Textile machinery parts wear because of contaminants with a diameter of about 3 μm contained in yarn, such as Al₂O₃ and SiO₂. Recently, yarn of poor quality containing high levels of contaminants may be used. Such yarn contains contaminants such as K₂O and CaO having a diameter of about 5 μm, which are slightly coarser than conventional contaminants. These coarse contaminants have been revealed to significantly affect the wear of textile machinery parts.

And improvement of wear resistance alone with the use of a carbide or a complex carbide of any one of Cr, Mo and V as described in PTL 1 to PTL 4 results in insufficient wear resistance, is unable to suppress wear due to coarse contaminants, and leads to high frequency of exchanging textile machinery parts.

In the case of weaving machine members of PTL 5 to PTL 13, warp yarn to be used herein is set by an air jet or water jet blowing. Hence, anti-corrosion characteristics should be taken into consideration and thus relatively expensive stainless steel is used.

However, textile machinery parts are mechanically activated to set warp yarn, and oil is added dropwise to parts that come into contact with the yarn, leading to less concern about anti-corrosion characteristics.

Accordingly, in the case of textile machinery parts, there is no need to apply expensive stainless steel to as many as a few thousand parts, as in the case of weaving machine members.

In addition, the toughness of weaving machine members is evaluated using bendability found by a bending test, as an indicator.

However, textile machinery parts have a sliding rate of several meters per second. Therefore a bending test is inappropriate for evaluation of toughness and may show inappropriately low toughness as textile machinery parts.

Here, textile machinery parts have extremely complicated forms of wear. Accordingly, there is a tendency to improve wear resistance by simply using a high-strength material while leaving the cause of the wear of sites unknown. The wear resistance of textile machinery parts may not be appropriately improved.

Moreover, the life of the materials of textile machinery parts is evaluated while these parts are mounted to an actual device and used under an actual use environment. This currently leads not only to take long time to select materials, but also to make selection of proper materials difficult.

Therefore, a steel sheet for textile machinery parts that can be manufactured at a low cost and is excellent in wear resistance and toughness is required.

The present invention has been achieved in view of these points, and an object of the present invention is to provide a steel sheet for textile machinery parts that can be manufactured at a low cost and is excellent in wear resistance and toughness, and a method for manufacturing the same.

A steel sheet for textile machinery parts according to claim 1 contains in mass %, C: 0.60% or more and 1.25% or less, Si: 0.50% or less, Mn: 0.30% or more and 1.20% or less, P: 0.03% or less, S: 0.03% or less, Cr: 0.30% or more and 1.50% or less, and Nb: 0.10% or more and 0.50% or less, with the balance being Fe and unavoidable impurities, wherein Nb-containing carbides having a particle size of 0.5 pun or more are present in the matrix at a density of 3000/mm² or more and 9000/mm² or less.

A steel sheet for textile machinery parts according to claim 2 is the steel sheet for textile machinery parts according to claim 1 containing in mass %, Ti: 0% (no Ti added) or more and 0.50% or less, and B: 0% (no B added) or more and 0.005% or less.

A steel sheet for textile machinery parts according to claim 3 is the steel sheet for textile machinery parts according to claim 1 or 2 contains in mass %, any one or more types of Mo: 0% (no Mo added) or more and 0.50% or less, V: 0% (no V added) or more and 0.50% or less, and Ni: 0% (no Ni added) or more and 2.0% or less.

A method for manufacturing a steel sheet for textile machinery parts according to claim 4 includes performing slab heat treatment after casting, wherein: when a heating temperature is designated as “T” in the slab heat treatment, and Y=2.43−6000/(T+273) and X=0.68 (Nb content)+0.10 (C content)−10^(Y) are employed,

the heating temperature of the slab heat treatment is determined depending on the C content and the Nb content, so that a Z value represented by the formula, Z value=3.24exp (4.61X), is 6 or more and 20 or less; and, upon casting, casting conditions are adjusted, so that the value of the average cooling rate during the cooling of the slab central part from the liquidus line temperature to the solidus line temperature is equal to or less than the Z value.

According to the present invention, the steel sheet for textile machinery parts contains in mass %, 0.60% or more and 1.25% or less C, 0.50% or less Si, 0.30% or more and 1.20% or less Mn, 0.03% or less P, 0.03% or less S, 0.30% or more and 1.50% or less Cr, and 0.10% or more and 0.50% or less Nb, with the balance being Fe and unavoidable impurities, and thus the steel sheet can be manufactured at a low cost.

Moreover, Nb-containing carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 3000/mm² or more, so that the wear resistance is good, and Nb-containing carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 9000/mm² or less, so that the toughness is good.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram schematically showing the construction of a melting and solidifying device for manufacturing simulated slabs.

FIG. 2 is a block diagram schematically showing the configuration of a yarn guide wear test.

FIG. 3 is a side view showing the shape of a test piece to be used for an impact test.

DETAILED DESCRIPTION

Hereafter, the configuration of an embodiment of the present invention is described in detail. In addition, the content of each element is expressed as mass %, unless otherwise specified.

The steel sheet for textile machinery parts contains 0.60% or more and 1.25% or less C (carbon), 0.50% or less Si (silicon), 0.30% or more and 1.20% or less Mn (manganese), 0.03% or less P (phosphorus), 0.03% or less S (sulfur), 0.30% or more and 1.50% or less Cr (chromium), and 0.10% or more and 0.50% or less Nb (niobium), with the balance being Fe (iron) and unavoidable impurities.

Further, the steel sheet for textile machinery parts preferably contains as necessary, 0% (no Ti added) or more and 0.50% or less Ti (titanium), and, 0% (no B added) or more and 0.005% or less B (boron).

Furthermore, the steel sheet for textile machinery parts preferably contains as necessary any one or more types of 0% (no Mo added) or more and 0.50% or less Mo (molybdenum), 0% (no V added) or more and 0.50% or less V (vanadium), and, 0% (no Ni added) or more and 2.0% or less Ni (nickel).

Carbon (C) is an element required for improving the strength of steel sheets, and the C content should be 0.60% or more in order to ensure the strength required for use in textile machinery parts. However, the C content of higher than 1.25% increases the amount of coarse undissolved carbides and causes deterioration in impact characteristics and the like. Therefore, the C content is determined to be 0.60% or more and 1.25% or less.

Silicon (Si) is added as a deoxidation material at the steelmaking stage, but if no Si is added, no poor deoxidation takes place. In addition, the high Si content causes deterioration in toughness, and the Si content of higher than 0.50% can make it impossible to ensure the toughness required for use in textile machinery parts. Therefore, the Si content is determined to be 0.50% or less (no Si added), and preferably 0.30% or less.

Manganese (Mn) is an element effective for improving the hardenability of steel, and hardenability cannot be sufficiently improved when the Mn content is less than 0.30%. However, Mn contained in a large amount, specifically, the Mn content of higher than 1.20% causes hardening and damages manufacturability and toughness. Therefore, the Mn content is determined to be 0.30% or more and 1.20% or less.

Both phosphorus (P) and sulfur (S) adversely affect toughness, and P and S contents are preferably as low contents as possible. Therefore, both P and S contents are determined to be 0.03% or less.

Chromium (Cr) is an element having an effect of improving the hardenability of steel, an effect of improving the strength of steel sheets, an effect of improving the wear resistance of steel sheets, and an effect of suppressing the coarsening of cementite upon annealing. And the Cr content should be 0.30% or more for Cr to exert each of the above effects. However, Cr may adversely affect such that Cr inhibits the solution treatment of cementite while heating is maintained in quenching treatment. Accordingly, the Cr content of higher than 1.50% can cause an increased amount of undissolved cementite upon quenching treatment. Therefore, the Cr content is determined to be 0.30% or more and 1.50% or less.

Niobium (Nb) forms extremely hard Nb-containing carbides in steel during a cooling process after casting, and contributes to improvement in wear resistance, and particularly resistance to abrasive wear. Further, Nb contributes to improvement in toughness by refining crystal grains upon quenching. The Nb content should be 0.10% or more for Nb to exert each of these effects. However, Nb added in a large amount results in excessive formation of Nb-containing carbides. Moreover, the Nb-containing carbides serve as a starting point of destruction and a crack propagation path, causing deterioration in toughness. Moreover, in order to ensure good toughness after thermal refining heat treatment in an application where the C content is at a relatively high level, it is important to keep the Nb content at 0.50% or less. Therefore, the Nb content is determined to be 0.10% or more and 0.50% or less.

Titanium (Ti) forms extremely hard Ti-containing carbides in steel, similar to Nb, during a cooling process after casting, and contributes to wear resistance. Further, titanium carbide (TiC) precipitated during hot rolling or cooling after re-solution treatment upon hot rolling or the like contributes to improvement in toughness by refining crystal grains upon quenching. Furthermore, binding force between Ti and N is strong, so that the feature is effective to prevent the formation of boron nitride (BN) upon addition of boron (B), and to exploit an effect of improving the hardenability of B. Therefore, Ti is preferably added as necessary, and the Ti content determined to be 0.01% or more is effective for Ti to exert each of the above effects. However, the Ti content of higher than 0.50% tends to cause deterioration in toughness, since Ti-based carbides are present in a large amount in the steel sheet. Hence, when Ti is contained, the Ti content is preferably determined to be 0.50% or less.

Boron (B) is an element effective for improving hardenability, and is preferably added as necessary. To obtain an effect of B, the B content should be 0.0003% or more. In addition, B's effect of improving hardenability is saturated when the B content is 0.005%. Therefore, when B is contained, the B content is preferably determined to be 0.005% or less.

Molybdenum (Mo) and vanadium (V) are both elements effective for improving toughness, and are preferably added as necessary. For Mo to exert an effect of improving toughness, the Mo content determined to be 0.1% or more is effective. However, Mo and V are relatively expensive elements, and thus the addition of Mo and V in excessive amounts increases the cost. Hence, when at least one type of Mo and V is contained, the Mo content and the V content are each preferably determined to be 0.50% or less.

Nickel (Ni) is an element effective for improving hardenability and low-temperature toughness, and is preferably added as necessary. For Ni to exert an effect of improving hardenability and an effect of improving low-temperature toughness, the Ni content determined to be 0.1% or more is effective. However, adding Ni in an excessive amount damages cost efficiency. Hence when Ni is added, the Ni content is preferably determined to be 2.0% or less.

To improve the wear resistance of the steel sheet for textile machinery parts made of the above chemical components, the effects of Nb-containing carbides are used. In addition, when Ti is contained, carbides of Ti are also effective for improving wear resistance. At this time, to ensure toughness for use in textile machinery parts, the particle size of carbides should be controlled.

Specifically, if the steel sheet for textile machinery parts that are final parts after thermal refining heat treatment has a metallographic structure in which Nb-containing carbides or Nb- and Ti-containing carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 3000/mm² or more and 9000/mm² or less, the wear resistance is improved and a harmful effect of deteriorating toughness can be avoided.

In addition the term “Nb-containing carbides” refers to hard carbides containing NbC as a principal component. The term “Nb- and Ti-containing carbides” refers to hard carbides (hereafter, these Nb-containing carbides and Nb- and Ti-containing carbides are referred to as hard carbides.) containing (Nb, Ti)C or the like as a principal component.

Whether or not precipitated particles contained in steel correspond to hard carbides can be confirmed by microscopic analysis such as EDX. Moreover, hard carbides confirmed in such a manner are each subjected to area measurement, the diameter of a perfect circle having the same area as that of each carbide is calculated and the diameter is designated as the particle size of the hard carbide.

When hard carbides in steel having a particle size of 0.5 μm or more are present at less than 3000/mm², the hard carbides' effect of improving wear resistance is insufficient and wear resistance sufficient for use in textile machinery parts may not be ensured. Furthermore, when hard carbides having a particle size of 0.5 μm or more are present at a level higher than 9000/mm², these hard carbides serve as a starting point of destruction and a crack propagation path, causing deterioration in toughness. Therefore, the steel sheet for textile machinery parts is specified such that hard carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 3000/mm² or more and 9000/mm² or less.

Next, a method for manufacturing the above steel sheet for textile machinery parts is described as follows.

The steel sheet for textile machinery parts is manufactured through casting, hot rolling and thermal refining heat treatment.

In the casting step, Nb-containing hard carbides, or, Nb- and Ti-containing hard carbides are precipitated in steel during a cooling process. To adjust the particle size and the density of precipitated hard carbides, the strict control of the C content, the Nb content, and the cooling rate upon casting is important.

Specifically, casting conditions are adjusted so that the value of an average cooling rate (° C./min) for cooling a slab central part from the liquidus line temperature to the solidus line temperature upon casting is equal to or less than a Z value represented by formula (1) Z value=3.24exp (4.61X). In general, since an extremely slow cooling rate (for example, 1° C./min or less) can adversely affect the productivity significantly, cooling is performed at a cooling rate of 5° C./min or more. In addition, in the formula (1), X=0.68 (Nb content)+0.10 (C content)−10^(Y) and Y=2.43−6000/(T+273) are employed, and the heating temperature of the slab heat treatment is designated as “T”.

The Z value represented by the formula (1) is an indicator representing the allowable upper limit (° C./min) of an average cooling rate for cooling a slab central part from the liquidus line temperature to the solidus line temperature upon casting based on the C content, the Nb content and the slab heating temperature. Furthermore, if the heating temperatures are the same, there is a tendency such that the higher the Z value, the coarser the hard carbides.

The particle size and the density of Nb-containing carbides to be precipitated in steel are also influenced by slab heating temperatures and the cooling rates thereafter in steps following the casting step, but are influenced more significantly in the cooling process in the casting step.

Moreover, the lower the average cooling rate for slabs in the casting step, the more progressed coarsening of hard carbides. When excessively coarsened hard carbides are present in slabs, coarse carbides serving as a starting point of destruction due to impact remain, even if re-solution treatment of hard carbides is attempted in slab heat treatment after casting. Therefore, the lower limit of the average cooling rate is preferably determined to be 5° C./min.

Furthermore, the higher the Nb content and the higher the C content in steel, the more facilitated coarsening of Nb-containing carbides. Since the Z value of higher than 20 easily causes deterioration in toughness, the Z value is determined to be 20 or less in order to ensure impact characteristics for use in textile machinery parts.

Through slab heat treatment, re-solution treatment of portions of Nb-containing carbides precipitated in slabs can be performed using the heating of slabs such as continuous casting slabs in hot rolling after casting.

Therefore, the higher the heating temperature T upon the slab heat treatment, the finer the hard carbides and the more improved the toughness.

In the slab heat treatment, similar to general hot rolling, the heating temperature T can be set at 1100° C. or higher and 1350° C. or lower.

Moreover, in the slab heat treatment, the time for maintaining heating (the time required for a slab central part to reach 50° C. or more below the steel material heating temperature T) is preferably 30 minutes or more and 240 minutes or less.

In addition, when slabs are subjected to heat treatment at a heating temperature T such that the Z value represented by the formula (1) is less than 6, the solution treatment of Nb-containing carbides can excessively proceed, causing deterioration in wear resistance. Therefore, the heating temperature T of the slab heat treatment is determined depending on the C content and the Nb content in steel, so that the Z value is 6 or more and 20 or less, and then casting conditions are adjusted on the basis of the Z value calculated based on the thus determined heating temperature T of the slab heat treatment.

In hot rolling, the temperature for finish rolling is determined to be 800° C. or higher and 900° C. or lower, for example, and the temperature for winding is determined to be 630° C. or lower, for example.

Furthermore, steel sheets after hot rolling are subjected to annealing and cold rolling.

Conditions for annealing can be adjusted as necessary. Specifically, heating is preferably maintained for 10 to 50 hours, for example, within a temperature range below the Ac₁ point at which austenite formation begins.

Furthermore, cold rolling is performed as necessary after annealing, and then annealing is performed again. In this manner, annealing and cold rolling may be repeated for several times. In addition, conditions for cold rolling can also be adjusted as necessary.

Moreover, after annealing and cold rolling performed as described above, the steel sheet has an annealed structure in which the matrix is ferrite phase, and is then subjected to thermal refining heat treatment such as quenching and tempering.

Thermal refining heat treatment is performed after processing of a steel sheet subjected to annealing and cold rolling into the shape of parts, the parts are thermally refined by quenching and tempering to have a hardness of 53 to 62 HRC, for example.

Moreover, thermal refining heat treatment is performed under general conditions except that temperatures of 1000° C. or lower are employed for solution treatment, so as not to disturb the previously adjusted distribution of hard carbides.

In addition, the metallographic structure of the steel sheet after thermal refining heat treatment is a hard carbide-containing martensitic structure.

Next, the effects of the above embodiment are explained as below.

The above steel sheet for textile machinery parts contains, in mass %, 0.60% or more and 1.25% or less C, 0.50% or less Si, 0.30% or more and 1.20% or less Mn, 0.03% or less P, 0.03% or less S, 0.30% or more and 1.50% or less Cr, and 0.10% or more and 0.50% or less Nb, with the balance being Fe and unavoidable impurities. Hence, unlike relatively expensive stainless steel according to conventional techniques as described in the above PTL 5 to PTL 13, the steel sheet can be manufactured at a low cost and is suitable for application to nearly thousands of textile machinery parts, for example.

Furthermore, the steel sheet for textile machinery parts has the above chemical components, and specifically Nb-containing carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 3000/mm² or more and 9000/mm² or less. Hence, the effect of improving wear resistance exerted by Nb-containing hard carbides can be ensured, and deterioration in toughness due to excessive formation of hard carbides can be prevented, so that the resulting wear resistance and toughness are good.

The steel sheet for textile machinery parts contains Ti as necessary, so that wear resistance and toughness can be improved by the effect of improving wear resistance and the effect of improving toughness, which are exerted by Ti-containing hard carbides.

Moreover, the steel sheet for textile machinery parts contains B as necessary, so that the hardenability can be improved. In addition, when B is contained, Ti is further contained so that the formation of BN (boron nitride) due to binding of B and N can be prevented, and thus B can easily exert its effect of improving hardenability.

Furthermore, the steel sheet for textile machinery parts contains at least one type of Mo, V and Ni, as necessary, so that toughness, hardenability, and low-temperature toughness can be improved.

According to a method for manufacturing the above steel sheet for textile machinery parts, a heating temperature T of slab heat treatment is determined depending on the C content and the Nb content so that the Z value represented by formula (1) is 6 or more. This can prevent excessive progression of the solution treatment of Nb-containing hard carbides upon slab heat treatment. Accordingly, the particle size or the density of hard carbides can be easily controlled, and a steel sheet for textile machinery parts having good wear resistance and good toughness resulting from the use of the effects of the hard carbides can be manufactured.

Furthermore, casting conditions are adjusted, so that the value of the average cooling rate when a slab central part is cooled from the liquidus line temperature to the solidus line temperature upon casting is equal to or less than the Z value calculated with the formula (1) using the above-determined heating temperature T This enables precipitation of the appropriate number of hard carbides having an appropriate size in steel. Accordingly, the particle size and the density of hard carbides can be easily controlled, and a steel sheet for textile machinery parts excellent in wear resistance and toughness can be manufactured.

Examples

Examples of the present invention are described as below.

Table 1 shows chemical components of steel sheets serving as base materials for textile machinery parts.

TABLE 1 Chemical composition (mass %) Steel C Si Mn P S Cr Nb A 0.92 0.32 0.77 0.012 0.008 1.01 0.33 B 0.83 0.31 0.42 0.015 0.006 0.53 0.19 C 0.66 0.19 1.03 0.022 0.010 0.89 0.17 D 1.02 0.05 0.91 0.019 0.003 1.44 0.36 E 1.18 0.24 0.58 0.025 0.016 0.37 0.27 F 0.79 0.42 1.06 0.016 0.013 0.96 0.46 G 0.53 0.63 0.82 0.014 0.005 0.50 — H 0.72 0.38 0.44 0.013 0.008 0.82 0.07 I 0.84 0.30 0.98 0.011 0.014 0.38 0.67 J 1.32 0.25 0.43 0.010 0.007 0.41 0.22 K 0.63 0.97 0.44 0.008 0.004 0.44 — L 0.91 0.26 0.61 0.006 0.008 16.30 — M 0.83 0.23 1.45 0.008 0.004 0.14 — 0.60~1.25 ≤0.50 0.30~1.20 ≤0.030 ≤0.020 0.30~1.50 0.10~0.50 Chemical composition (mass %) Steel Ti Mo V Ni B Category A — — — — — Example of the invention B 0.02 — — — 0.0010 Example of the invention C — — 0.18 — — Example of the invention D 0.14 0.29 — — — Example of the invention E 0.21 — — 0.89 — Example of the invention F — — — 0.51 — Example of the invention G — — — — — Comparative example H — — — — — Comparative example I — — — 0.51 — Comparative example J — — — — — Comparative example K — 0.15 0.14 — — Comparative example L — — — — — Comparative example M — — — — — Comparative example ≤0.50  ≤0.50  ≤0.50  ≤1.0   0.0005~0.005

Each steel slab shown in Table 1 was melt-formed, and then 30 kg of a steel ingot for melting and solidification experiments was cut. Next, the steel ingot was melted in a crucible furnace to produce molten steel, and then the cooling rate upon solidification was controlled, thereby obtaining a solidified ingot simulating a slab obtained by varying the cooling rate upon casting.

Specifically, solidified ingots were produced using a melting/solidification apparatus 1 shown in FIG. 1.

First, a steel block was melted by the heat of a heater 4 within a cylindrical crucible 3 covered with a heat insulating material 2, thereby obtaining a molten steel 5.

The cylindrical crucible 3 is placed on a stage 7 capable of moving up and down through firebricks 6. Then, from the state of the molten steel temperature of 1700° C., the stage 7 was moved down to transfer the cylindrical crucible 3 accommodating the molten steel 5 into a cooling zone where a water-cooled coil 8 was placed, to solidify the molten steel 5.

Upon cooling in the cooling zone, the temperatures of the molten steel 5 and the solidified ingot resulting from solidification of the molten steel 5 were monitored by a thermocouple 9 placed at the center of the cylindrical crucible 3, and the descending speed of the stage 7, the heat amount of the heater 4, and the heat reduction amount of the water cooling coil 8 were adjusted, so that the average cooling rate while cooling from the liquidus line temperature to the solidus line temperature was a predetermined value ranging from 5° C./min or more to 20° C./min.

The thus obtained solidified ingot was a product simulating the slab resulting from the control of the rate of cooling the slab central part upon casting. Hereafter, the solidified ingots are designated as simulated slabs, and the average cooling rate of the above cooling is considered to be the average cooling rate while cooling of the slab central parts from the liquidus line temperature to the solidus line temperature upon casting.

Simulated slabs were each treated in order of hot rolling, annealing, cold stretching, annealing and thermal refining heat treatment, thereby manufacturing impact test pieces having a sheet thickness of 1.8 mm.

Moreover, these steel sheets were further repeatedly subjected to cold stretching and annealing, thereby manufacturing wear test pieces having a sheet thickness of 0.2 mm.

Furthermore, impact test pieces and wear test pieces were thermally refined by thermal refining heat treatment to have a thermal refining hardness of 62 HRC.

In addition, hot rolling was performed by keeping heating temperatures of 1250° C. to 1350° C. for 60 minutes, followed by finishing at a finishing temperature of 850° C. and winding at a winding temperature of 590° C., thereby obtaining a hot-rolled sheet having a hot-rolled sheet thickness of 3.5 mm (adjusted to be 3.0 mm by grinding processing). Annealing was performed by heating the sheet to 690° C. and then keeping it at the temperature for 18 hours.

The thermal refining heat treatment was performed by carrying out heat treatment at 830° C. for 15 minutes, and oil quenching at 60° C. Thus thermal refining materials having a thermal refining hardness of 740 HV was obtained according to the compositions. All thermally refined materials were each found to have a hardness within the range of 740 HV+15 HV as measured with a Vickers hardness tester.

Here, before thermal refining heat treatment, the cross sections (L section) of a steel sheet, which were parallel to the rolling direction and the direction of sheet thickness, were subjected to mirror polishing, subjected to etching with Murakami reagent (alkaline solution of red prussiate of potash), and then observed under a confocal scanning microscope. Moreover, the images were processed, and then the quantity of Nb-containing carbides (hard carbides) existing in the area of a visual field was measured, thereby calculating the density of the carbides existing in the area.

Regarding Nb-containing hard carbides, particles existing in an observance area of 90*60 μm*20 visual fields and having a particle size of 0.5 μm or more were counted, and then each value was converted to the number per 1 mm² based on the results.

Here, the particle size refers to a diameter of the area of a circle corresponding to the particle area. Particles having a particle size of 0.5 μm or more were picked up through image processing.

FIG. 2 schematically shows a method for testing yarn guide wear. After thermal refining heat treatment, a strip-shaped test piece 11 having a sheet thickness of 0.2 mm, the length in a lengthwise direction of 60 mm, the length in a width direction of 20 mm was fixed with a jig, and then a load of 2N was applied using a weight, preparing a condition where friction exists between the surface of the test piece 11 and a yarn 12.

Furthermore, the yarn guide wear test was conducted using a 110-decitex domestic polyester spun yarn, a feed rate of 30 m/min, and a friction distance of 10000 m while adding dropwise silicone oil for a sewing machine to the contact surface between the test piece 11 and the yarn 12.

Then, the wear track depth of each test piece 11 was measured by a laser microscope, pieces having a comparative wear amount of less than 0.6*10-7 mm³/Nm were determined to have acceptable wear resistance as a steel sheet for textile machinery parts. In addition, on the friction surfaces of the test pieces 11, streaked wear traces similar to those observed on textile machinery parts recovered from the market were observed.

FIG. 3 shows the shape of an impact test piece. When a vertical direction (T direction) with respect to the rolling direction was designated as a lengthwise direction, a test piece 21 in the impact test was produced to have a sheet thickness of 1.8 mm, the length in the lengthwise direction of 55 mm, and the length in a width direction of 10 mm, and a U notch 22 with R1 mm at the central part in the lengthwise direction.

Then, a Charpy impact test was conducted at normal temperature for the test piece 21, so as to find an impact value resulting from the impact direction shown with an arrow. A test piece found to have a 2-mm U notch impact value of 5 J·cm⁻² or more was determined to have toughness (impact characteristics) acceptable as a steel sheet for textile machinery parts.

Table 2 shows slab heat treatment conditions, the results of measuring the density of hard carbides, the results of the yarn guide wear test, and the results of the impact test.

TABLE 2 Number of carbide Hard carbide with a Slab heat treatment condition particle size of Wear resistance Heating 0.5 μm or more Comparative Impact characteristics temperature Cooing rate (number of hard wear amount Accepted/ Impact value Accepted/ Test No. Steel T/° C. ° C./min Z value carbides/mm²) * 10⁻⁷ mm³/Nm rejected J · cm⁻² rejected 1 A 1250 10 12.08 6038 0.33 Accepted 7.1 Accepted 2 1250 7 12.08 6156 0.32 Accepted 7.0 Accepted 3 1250 6 12.08 6354 0.31 Accepted 6.8 Accepted 4 1250 5 12.08 6749 0.30 Accepted 6.4 Accepted 5 1350 7 10.86 6096 0.33 Accepted 7.1 Accepted 6 B 1300 15 7.12 2783 0.79 rejected 9.9 Accepted 7 1300 6 7.12 3116 0.55 Accepted 8.8 Accepted 8 1300 5 7.12 3295 0.52 Accepted 8.3 Accepted 9 1250 5 7.47 4059 0.40 Accepted 6.8 Accepted 10 1250 6 7.47 4026 0.40 Accepted 6.8 Accepted 11 C 1350 15 5.83 2883 0.71 rejected 9.8 Accepted 12 1300 5 6.19 3204 0.45 Accepted 8.8 Accepted 13 1250 5 6.49 3441 0.42 Accepted 8.2 Accepted 14 D 1250 10 13.90 7058 0.23 Accepted 6.0 Accepted 15 1350 10 12.49 7024 0.23 Accepted 6.0 Accepted 16 1250 6 13.90 7469 0.22 Accepted 5.7 Accepted 17 E 1250 10 11.28 5023 0.41 Accepted 5.5 Accepted 18 1250 8 11.28 5060 0.40 Accepted 5.4 Accepted 19 1350 6 11.28 5248 0.39 Accepted 5.2 Accepted 20 F 1250 10 15.09 8287 0.15 Accepted 7.7 Accepted 21 1250 6 15.09 8845 0.14 Accepted 7.2 Accepted 22 1300 6 14.38 8746 0.15 Accepted 7.3 Accepted 23 G 1250 10 3.59 0 1.53 rejected 9.0 Accepted 24 1250 6 3.59 0 1.45 rejected 8.6 Accepted 25 H 1250 6 4.88 2016 0.84 rejected 8.8 Accepted 26 I 1250 20 33.80 10084 0.09 Accepted 3.3 rejected 27 1350 15 30.38 10148 0.09 Accepted 3.2 rejected 28 J 1250 20 10.29 2782 0.84 rejected 1.5 rejected 29 1250 15 10.29 2892 0.81 rejected 1.4 rejected 30 K 1250 8 3.76 0 1.33 rejected 3.4 rejected 31 1250 7 3.76 0 1.28 rejected 3.3 rejected 32 1250 6 3.76 0 1.30 rejected 3.6 rejected 33 L 1250 7 4.27 0 1.18 rejected 2.3 rejected 34 1250 5 4.27 0 1.28 rejected 2.5 rejected 35 M 1250 8 4.12 0 1.38 rejected 2.8 rejected 36 1350 6 3.70 0 1.44 rejected 2.7 rejected

As shown in Table 2, the examples of the present invention were manufactured by using the predetermined chemical components, determining the heating temperature T of slab heat treatment so that the Z value represented by formula (1) was 6 or more and 20 or less, and, controlling the cooling rate of a simulated slab, so that the value of average cooling rate was equal to or less than the Z value while the central part of the simulated slab was cooled from the liquidus line temperature to the solidus line temperature. The examples were found to have the adjusted number of hard carbides with a particle size of 0.5 μm or more, ranging from 3000 to 9000/mm². As a result, the examples of the present invention were excellent in both wear resistance and toughness and had high impact characteristics.

On the other hand, comparative examples No. 6, No. 28 and No. 29 had the average cooling rates of slabs higher than the Z value, and thus the number of hard carbides was less than 3000/mm², and the wear resistance was insufficient.

Comparative example No. 11 had the Z value of less than 6, and the value of the cooling rate of higher than the Z value, so that the number of hard carbides was less than 3000/mm², and the wear resistance was insufficient.

No. 23, No. 24 and Nos. 30 to 36 contained no Nb, so that no hard carbides (Nb-containing carbide) were present, and the wear resistance was significantly low.

Comparative example No. 25 had a low Nb content and a low C content, so that the number of hard carbides was less than 3000/mm², and the wear resistance was insufficient.

Comparative examples No. 26 and No. 27 had excessive Nb contents, so that hard carbides remained in excessive amounts and the impact characteristics were significantly decreased.

The present invention can be used for textile machinery parts such as latch needles, needle plates, sinkers, selectors, and jacks to be used for knitting machines. 

1. A steel sheet for textile machinery parts, comprising in mass %, C: 0.60% or more and 1.25% or less, Si: 0.50% or less, Mn: 0.30% or more and 1.20% or less, P: 0.03% or less, S: 0.03% or less, Cr: 0.30% or more and 1.50% or less, and Nb: 0.10% or more and 0.50% or less, with the balance being Fe and unavoidable impurities, wherein Nb-containing carbides having a particle size of 0.5 μm or more are present in the matrix at a density of 3000/mm² or more and 9000/mm² or less.
 2. The steel sheet for textile machinery parts according to claim 1, comprising in mass %, Ti: 0% (no Ti added) or more and 0.50% or less, and B: 0% (no B added) or more and 0.005% or less.
 3. The steel sheet for textile machinery parts according to claim 1, comprising in mass %, any one or more types of Mo: 0% (no Mo added) or more and 0.50% or less, V: 0% (no V added) or more and 0.50% or less, and Ni: 0% (no Ni added) or more and 2.0% or less.
 4. A method for manufacturing a steel sheet for textile machinery parts, comprising performing slab heat treatment after casting, wherein: when a heating temperature is designated as “T” in the slab heat treatment, and Y=2.43−6000/(T+273) and X=0.68 (Nb content)+0.10 (C content)−10^(Y) are employed, the heating temperature of the slab heat treatment is determined depending on the C content and the Nb content, so that a Z value represented by the formula, Z value=3.24exp (4.61X), is 6 or more and 20 or less; and, upon casting, casting conditions are adjusted, so that the value of the average cooling rate during the cooling of the slab central part from the liquidus line temperature to the solidus line temperature is equal to or less than the Z value.
 5. The steel sheet for textile machinery parts according to claim 2, comprising in mass %, any one or more types of Mo: 0% (no Mo added) or more and 0.50% or less, V: 0% (no V added) or more and 0.50% or less, and Ni: 0% (no Ni added) or more and 2.0% or less. 